Avoid macOS notifications

Hi All,

I got a tricky challenge which I has not been able to find a solution for yet?!

Our users receive a popup notification telling that a new macOS-update is available now for a new macOS version HIGH SIERRA, and if they like to install and upgrade now...

This is a problem because our server is not upgraded yet, so it will result in some problems if the user clients suddenly accept the upgrade and will then use a higher version than the company server is...

In System Settings -> App Store there is NOT any checkmark in "Install macOS updates" - but this does not prevent the popup notification from showing up... so how do I avoid this popup notification from showing for macOS upgrades only (the install app / program updates are checked okay, and allowed, ONLY the macOS popups that we want to avoid showing to the user clients - but how to fix this specific problem)?!

Thanks in advance.

Kind regards,

Mr ITee